Posture is the position of the body when one is sitting or standing or walking. In fact the word posture includes the way one holds oneself while doing any activity including lying down or sleeping. I assume that this question refers to the sitting posture because sitting position is what most people find themselves in for most part of waking hours. Especially in an urban setting in this modern, digital world.
However if one finds that their sitting posture is bad, it is usually bad during all activity listed above. This cannot be corrected just by learning or understanding what is the right posture recommended for each activity. The problem lies much deeper and once that deep problem is addressed, one can seek out and learn or even experiment with different postures to find out which works best.
So what is this deeper problem? It is that one is usually absent from the body for most part of the day. The body slumps into a bad posture because of this absence. You may ask, if one is absent from the body, where are they present? They are present in their thoughts. Completely lost in their thoughts or completely lost in one or the other form of mental activity constantly.
The first step I recommend is to learn to switch off this incessant mental chatter and allow the mind to rest quietly in the present moment. I mention present moment because the mental activity either concerns the past or the future. It is not easy to be able to quiet the mind at first because of the conditioning it has gone through while growing up. When the mind quietens, one would find themselves being present in their body. This is called an Aware state of being, or a Grounded state of being, or being in the Present moment of thought free awareness.
There are many practices that you can use as a portal to switch to this state of quiet awareness from the usual restless, distracted and thinking state of mind. Some of the effective practices that I teach are, Focussed Gazing and Conscious breathing, Deep Crossing and other gently body movements in sync with the breath that I refer to as Energy Movements, Gentle flexing and stretching movements that I refer to as SelvanS 30 Yoga, etc.
You can find resources to learn all of these on my website or in the various articles that I have published on my Quora Profile or in my book titled Inner Alchemy.
In the beginning it is only possible to remain in this aware state for a few seconds. Consistent practice would gradually improve body awareness and you may find that the body posture automatically corrects itself.